Catalysts for automobiles have a function to decompose and remove hydrocarbons (HCs), nitrogen oxide (NOx), and carbon monoxide (CO), which are harmful components in exhaust gas. Such a catalyst is produced by using, as a substrate, an inorganic material such as cordierite or a metal formed into a honeycomb shape. The harmful components are decomposed and removed when exhaust gas passes through the inside of the catalyst. To increase the contact efficiency of the catalyst with exhaust gas, the surface of the substrate is coated with a porous inorganic material, and furthermore, a trace amount of a noble metal is supported as an active component on a surface layer portion of the porous inorganic material. A platinum group metal such as platinum, palladium, or rhodium is used as the noble metal.
Recently, in order to rapidly treat exhaust gas at the ignition of an automobile engine, there has been a need for activation of the catalyst even at low temperatures. In this respect, improvement of purification performance (low-temperature activity) has been attempted by increasing the amount of the noble metal. However, the increase in the amount of the expensive noble metal leads to increase in costs of automobiles, and is disadvantageous to consumers. Hence, there has been a need for improvement in purification performances without increase in the amount of the noble metal.
In the meantime, a platinum hydroxide polymer is a hydroxide in which platinum atoms are cross-linked with about several to several tens of oxygen atoms. For preparation of the platinum hydroxide polymer, hexahydroxoplatinic acid (H2Pt(OH)6) is used as a raw material. The platinum hydroxide polymer is formed as follows. Specifically, hexahydroxoplatinic acid is caused to be present in the form of a hydroxo complex by being dissolved in a strong acid solution, and then the complex is subjected to influence of protons from the acid to thereby form reactive monomers and, in turn, to cause a polymerization reaction. Since the polymerization reaction proceeds rapidly, it is difficult to perform control during the reaction. However, when the acid concentration, the reaction temperature, and the raw material concentration are set to satisfy specific conditions, the polymer turns into a metastable state under which the polymer has a degree of polymerization depending on the conditions, and under which the reaction is temporarily stopped. In addition, since the polymerization reaction is irreversible, the polymerization state can be maintained even when the temperature is lowered from the reaction temperature. The number of platinum atoms contained in a polymer varies among platinum hydroxide polymers prepared as described above, depending on the degree of polymerization. Hence, it is possible to use this polymer, for example, as a precursor material for controlling a particle diameter of the noble metal for an exhaust gas catalyst or the like.
However, the polymerization reaction of the platinum hydroxide polymer gradually proceeds even at room temperature. Hence, not only does the particle diameter of the prepared platinum hydroxide polymer increase as compared with the initial particle diameter thereof, but also turbidity or precipitation occurs in the solution, resulting in decrease in platinum concentration in the solution. For this reason, it is difficult to store a solution containing a platinum hydroxide polymer at room temperature for long periods. In order to industrially use a solution containing a platinum hydroxide polymer as a material for an exhaust gas catalyst or the like, it is important to keep the metastable state for a long period.
In addition, suppose a case in which, after the control of the degree of polymerization, ion-exchanged water or the like is used to dilute the solution containing the platinum hydroxide polymer to thereby prepare a solution having a desired platinum concentration. In such a case, turbidity or precipitation occurs in the solution, because the platinum hydroxide polymer whose particle diameter has been controlled cannot be present stably in the solution. For this reason, in order to prepare a catalyst in which a platinum hydroxide polymer is supported, it is necessary to use a solution that is highly acidic and has a high platinum concentration. Hence, it is difficult to make the catalyst support a desired amount of platinum. In addition, the support on which platinum is supported needs to have acid resistance.
Japanese Patent Application Publication No. 11-92150 discloses stabilization of a platinum solution prepared by dissolving hexahydroxoplatinic acid into nitric acid. However, the target of the stabilization is not a solution containing a platinum hydroxide polymer. In addition, the platinum solution is diluted not with ion-exchanged water, but with nitric acid. Hence, the above-described problem cannot be solved.